The long-term goal of this project is to further our understanding of how the auditory system forms pitch percepts and how pitch or fundamental frequency is used to separate sounds coming from different sources. The project has 3 main aims, all of which combine psychophysical experiments with computational modeling. The first aim is to elucidate how the pitch of single harmonic complex sounds is coded in normal and impaired hearing. We hypothesize that the reduced availability of resolved harmonics, in conjunction with a reduction in the coding accuracy of individual harmonics, can predict many difficulties experienced by hearing-impaired listeners in more complex acoustic environments. The second aim is to continue our search for the mechanisms underlying our ability to perceive multiple pitches and to hear out 1 pitch in the presence of other competing complexes. By studying carefully controlled situations where the stimuli either completely overlap in spectrum or do not overlap at all, we are able to control the degree to which individual harmonics within the complexes are resolved. These situations serve as models for the more complex situations, where multiple partially-overlapping sounds combine to form our everyday acoustic environment. The third aim takes a more applied approach by examining the role of pitch and fundamental frequency in speech and music perception. Using simulations of cochlear-implant processing we will elucidate the mechanisms involved in providing a benefit under conditions of residual hearing (acoustic plus electric stimulation) and assess the degree to which the benefit can be ascribed to improved pitch information. We will also begin a systematic analysis of the presence (or lack thereof) of resolved harmonics in everyday acoustic situations, such as speech in the presence of competing talkers and music in small ensembles. The data from these analyses will provide strong constraints for models of complex pitch perception that rely to varying degrees on the presence of resolved harmonics. Understanding how pitch is coded and utilized in the normal and impaired auditory system should have important implications for the design and development of auditory prostheses, such as cochlear implants, and for source segregation algorithms within automatic speech recognition systems.